Preemptive nematic order, pseudogap, and orbital order in the iron pnictides

TL;DR

This paper develops a microscopic model for iron pnictides showing that nematic order preempts magnetic order, leading to complex phase diagrams with pseudogap behavior and orbital order, aligning well with experimental observations.

Contribution

It introduces a detailed low-energy effective model explaining nematic, magnetic, and orbital orders in iron pnictides from a microscopic perspective.

Findings

Nematic order preempts magnetic stripe order in iron pnictides.

Phase diagrams feature split magnetic and nematic tri-critical points.

Nematic transition can induce pseudogap behavior and orbital order.

Abstract

Starting from a microscopic itinerant model, we derive and analyze the effective low-energy model for collective magnetic excitations in the iron pnictides. We show that the stripe magnetic order is generally preempted by an Ising-nematic order which breaks $C_{4}$ lattice symmetry but preserves O(3) spin-rotational symmetry. This leads to a rich phase diagram as function of doping, pressure, and elastic moduli, displaying split magnetic and nematic tri-critical points. The nematic transition may instantly bring the system to the verge of a magnetic transition, or it may occur first, being followed by a magnetic transition at a lower temperature. In the latter case, the preemptive nematic transition is accompanied by either a jump or a rapid increase of the magnetic correlation length, triggering a pseudogap behavior associated with magnetic precursors. Furthermore, due to the distinct orbital character of each Fermi pocket, the nematic transition also induces orbital order. We compare our results to various experiments, showing that they correctly address the changes in the character of the magneto-structural transition across the phase diagrams of different compounds, as well as the relationship between the orthorhombic and magnetic order parameters.